The present invention relates to ballast circuits for starting gas discharge lamps, and, more particularly, to an improved rapid-start ballast circuit for a fluorescent lamp that produces high frequency oscillations superimposed on the AC voltage wave applied across the lamp for more efficient and reliable starting thereof.
A fluorescent lamp is a well known light source which typically consists of an elongate glass envelope having an interior wall coated with a suitable phosphor, such as calcium halophosphate, rare earth aluminate, zinc silicate or calcium tungsten, and having a cathode at each end of the envelope for application of an AC voltage across the lamp. The envelope also contains a quantity of an ionizable material such as mercury, and a low pressure of fill gas such as argon, krypton, neon or a mixture of these gases. The lamp is started by applying a large AC voltage supplied by a ballast circuit across the cathodes of the lamp to create a flow of electrons or an arc between the cathodes. The electron flow results in the ionization of the mercury atoms and the atoms of the fill gas which produces ultraviolet radiation. The ultraviolet radiation impinges upon the phosphor on the wall of the lamp which generates visible light in response thereto.
Creating the arc between the lamp cathodes and establishing ionization of the fill gas atoms with conventional ballast circuits can be a problem. Several cycles of the AC starting voltage and several starting attempts are often needed before the lamp lights, resulting in the familiar flickering of the lamp before it is fully illuminated. Starting aids are often employed to improve the capacitive coupling between the lamp and the mounting fixture to improve the starting efficiency and reliability. One technique is to reduce the spacing between the lamp and its mounting fixture, but this can reduce effective light output and cause undue heating of the fixture. Other starting aids include painting or otherwise affixing conductor strips on the lamp or providing an expensive tin oxide coating on the interior of the lamp to improve the capacitive coupling between the lamp and its mounting fixture.
It is accordingly an object of the present invention to provide an improved ballast circuit for a fluorescent lamp.
An additional object is to provide an improved ballast circuit of the above-character, which is capable of starting a fluorescent lamp in rapid-start fashion.
A further object is to provide a rapid-start ballast circuit of the above-character, wherein lamp starting is effected in an efficient and reliable manner without resort to special lamp starting aids.
Another object is to provide a ballast circuit of the above-character, which is simple in construction, inexpensive to manufacture, and reliable over a long service life.
Other objects of the invention will in part be obvious and in part appear hereinafter.